1. Field of the Invention
Silica, while abundant in many natural sources, is rarely found there in a highly pure amorphous form. Rather when found in natural sources of inorganic origin, it is typically of crystalline form and contains inorganic impurities. Conversely, when found in biogenic matter, the silica is amorphous but typically contains organic impurities, including various hydrocarbons. The invention disclosed and claimed herein relates to a process for recovery of highly pure amorphous silica from biogenic materials, particularly rice hulls, which are rich in amorphous silica content.
2. Description of Related Art
Means have been described for the product of amorphous silica from inorganic materials, but such means are expensive, hazardous, consume substantial energy and involves polluting chemicals. In general, such means involves a high temperature reacting finely divided inorganic source material with a strong halogen, such as chlorine or fluorine, to create silicon halides such as silicon tetrachloride or silicon tetrafloride, and subsequently oxidizing the halide to recover silica.
Means have also been described for recovery of silica from biogenic source material, in particular rice hulls. U.S. Pat. No. 3,125,043 to Gravel describes one such process. In Gravel a bed of rice hulls is subjected to up-flow of air at elevated temperature (between approximately 500-800xc2x0 C.). The resulting ash is constituted of amorphous silica which still contains from 2-10% impurities, which remains far too impure for many uses. While not stated in Gravel, applicant""s own experience suggests that the remaining impurities are primarily composed of a carbonaceous residue which cannot readily be made volatile by oxidation, because as Gravel states, a somewhat increased temperature merely tends to cause the impurities to be included within the silica particles. While the carbonaceous residue might be oxidized by a substantially higher temperature, such temperature would cause the resulting silica to be of crystalline rather than amorphous form.
Another attempt to produce silica from biogenic sources, to produce silicon, is seen in U.S. Pat. No. 4,214,920 to Amick et al. There rice hull ash is first treated with strong mineral acid and then coked (subjected to elevated temperature in a non-oxidizing environment). The result is a black, non-agglomerating form of silica having substantial amounts of carbon as an impurity. To produce silicon from such material, high purity silica must be added and the mixture thermally reduced to silicon at a temperature of about 1400xc2x0 C.
Another way to extract silica from biogenic sources is illustrated in Chinese Pat. No. 1063087 to Xianhe Yang. There rice hulls or rice hull ash is placed in a pressure containing vessel where they are subjected to strong concentrations of nitric acid and hydrogen peroxide at elevated temperature. The resulting residue is washed, to neutral pH, with de-ionized water, to remove remaining chemical residue.
Yet another way to extract silica from biogenic material, such as rice hulls, is demonstrated in U.S. Pat. Nos. 1,293,008 and 5,833,940. In these patents rice hulls or ash thereof is reacted with carbonates or hydroxides of alkali metal, such as sodium or potassium, to form a soluble silicate solution, from which silica may be subsequently precipitated.
Each of the above described methods produce amorphous silica which has substantial quantities of impurities, particularly carbon-based compounds and involves substantial expense and use of substantial quantities of hazardous chemicals, or both. As a result of the undesirable characteristics of processes to extract silica from biogenic materials containing silica, such as rice hulls, have traditionally been considered to be a waste product, of little or no utility. Large quantities cannot be left to rot near milling facilities. They are too bulky to transport and dispose economically at designated disposal sites. Several jurisdictions prohibit open burning of such materials. Only limited attempt has been made to burn such materials for power generation, since the combustion by-products tend to be abrasive. The invention described and claimed herein addresses these issues by providing an economical, energy efficient process to convert what is presently considered a waste disposal problem to highly valuable amorphous silica of very high purity.
The process herein described and claimed is principally characterized by two steps, soaking a biogenic material which contains a recoverable amount amorphous silica, particularly rice hulls, in a solution containing an oxidizing agent, such as hydrogen peroxide or peracetic acid for an effective length of time at an effective temperature and then burning the hulls at an elevated temperature (above approximately 500xc2x0 C., but substantially below the melting point of silica). While no claim is made of the precise manner or effect in which soaking a biogenic source material with oxidizing solution reacts with the particular molecules of said material, it is believed that such step causes the oxidizing agent to act as xe2x80x9cchemical scissorsxe2x80x9d, which xe2x80x9ccutxe2x80x9d long hydrocarbon molecules of the biogenic material, such as lignin and cellulose, into shorter, more volatile molecules, which are subsequently consumed (driven off as heated gases) during the step of burning, Following the step of burning non-oxidized impurities (which are typically comprised of small amounts of non-volatile, inorganic minerals, such as various magnesium and calcium compounds) which remain with the silica may then be substantially removed from the amorphous silica by rinsing with water, mild acid solutions. Prior to the step of soaking the biogenic material in an oxidizing solution, it may be cleaned by screening and washing, and/or softened by pre-soaking in liquid, which may be of elevated temperature.